Thermoelastic and superelastic ni-ti-w alloy

ABSTRACT

A radiopaque nitinol stent for implantation in a body lumen is disclosed. The stent is made from a superelastic alloy such as nickel-titanium or nitinol, and includes a ternary element including tungsten. The added tungsten in specified amounts improve the radiopacity of the nitinol stent comparable to that of a stainless steel stent of the same strut pattern coated with a thin layer of gold. Furthermore, the nitinol stent has improved radiopacity yet retains its superelastic and shape memory behavior and further maintains a thin strut/wall thickness for high flexibility.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of co-pending application U.S. Ser.No. 12/685,991, filed Jan. 12, 2010, which is a continuation of U.S.Ser. No. 10/917,701, filed Aug. 13, 2004, now U.S. Pat. No. 7,658,760,which is a continuation of U.S. Serial. No. 10/406,999, filed Apr. 3,2003, now U.S. Pat. No. 6,776,795, which is a continuation of U.S.Serial. No. 09/752,785 filed Dec. 28, 2000, now U.S. Pat. No. 6,569,194,the entireties of all of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

The present invention generally relates to self-expanding endoprostheticdevices including self-expanding intraluminal vascular grafts, generallycalled stents. More precisely, the present invention relates to stentsmade of radiopaque nitinol that can be used in essentially any bodylumen.

Stents are typically implanted in a body lumen, such as carotidarteries, coronary arteries, peripheral arteries, veins, or othervessels to maintain the patency of the lumen. These devices arefrequently used in the treatment of atherosclerotic stenosis in bloodvessels especially after percutaneous transluminal angioplasty (PTA) orpercutaneous transluminal coronary angioplasty (PTCA) procedures withthe intent to reduce the likelihood of restenosis of a vessel. Stentsare also used to support a body lumen, tack-up a flap or dissection in avessel, or in general where the lumen is weak to add support.

During PTCA procedures it is common to use a dilation catheter to expanda diseased area to open the patient's lumen so that blood flows freely.Despite the beneficial aspects of PTCA procedures and its widespread andaccepted use, it has several drawbacks, including the possibledevelopment of restenosis and perhaps acute thrombosis and sub-acuteclosure. This recurrent stenosis has been estimated to occur inseventeen to fifty percent of patients despite the initial PTCAprocedure being successful. Restenosis is a complex and not fullyunderstood biological response to injury of a vessel which results inchronic hyperplasia of the neointima. This neointimal hyperplasia isactivated by growth factors which are released in response to injury.Acute thrombosis is also a result of vascular injury and requiressystemic antithrombotic drugs and possibly thrombolytics as well. Thistherapy can increase bleeding complications at the catheter insertionsite and may result in a longer hospital stay. Sub-acute closure is aresult of thrombosis, elastic recoil, and/or vessel dissection.

Several procedures have been developed to combat restenosis andsub-acute or abrupt closure, one of which is the delivery and implantingof an intravascular stent. Stents are widely used throughout the UnitedStates and in Europe and other countries. Generally speaking, the stentscan take numerous forms. One of the most common is a generallycylindrical, hollow tube that holds open the vascular wall at the areathat has been dilated by a dilation catheter. One highly regarded stentused and sold in the United States is known under the tradename ACSMulti-Link Stent, which is made by Advanced Cardiovascular Systems,Inc., Santa Clara, Calif.

In expandable stents that are delivered with expandable catheters, suchas balloon catheters, the stents are positioned over the balloon portionof the catheter and are expanded from a reduced diameter to an enlargeddiameter greater than or equal to the inner diameter of the arterialwall by inflating the balloon. Stents of this type can be expanded to anenlarged diameter by deforming the stent, by engagement of the stentwalls with respect to one another, and by one way engagement of thestent walls together with endothelial growth onto and over the stent.

Examples of intravascular stents can be found in U.S. Pat. No. 5,292,331(Boneau); U.S. Pat. No. 4,580,568 (Gianturco); U.S. Pat. No. 4,856,516(Hillstead); U.S. Pat. No. 5,092,877 (Pinchuk); and U.S. Pat. No.5,514,154 (Lau et al.), which are incorporated herein by reference intheir entirety.

The problem with some prior art stents, especially those of the balloonexpandable type, is that they are often stiff and inflexible. Theseballoon expandable type stents are commonly formed from stainless steelalloys and the stents are constructed so that they are expanded beyondtheir elastic limit. As a result, such stents are permanently deformedby the inflation balloon beyond their elastic limits to hold open a bodylumen and thus maintain patency of that body lumen. There are severalcommercially available balloon expandable stents that are widely used;they are generally implanted in the coronary arteries after a PTCAprocedure mentioned earlier.

Stents are often times implanted in vessels that are closer to thesurface of the body, such as in the carotid arteries in the neck or inperipheral arteries and veins in the leg. Because these stents are soclose to the surface of the body, they are particularly vulnerable toimpact forces that can partially or completely collapse the stent andthereby block fluid flow in the vessel. Other forces can impact balloonexpandable stents and cause similar partial or total vessel blockage.For instance, under certain conditions, muscle contractions might alsocause expandable stents to collapse partially or completely. Thecollapse occludes the lumen and restricts blood flow in the vessel inwhich they are implanted.

Since balloon expandable stents are plastically deformed, once collapsedor crushed they remain so, permanently blocking the vessel. Thus,balloon expandable stents under certain conditions might pose anundesirable condition for the patient.

Self-expanding stents as the name implies self-expand through theproperties of the material constituting the stent. The inflation forceof a balloon catheter is usually not necessary to deploy this kind ofstent.

Important applications including those mentioned above have prompteddesigners to seek out superelastic shape memory alloys to exploit thematerials' properties in their self-expanding stents. Examples ofapplying superelastic nickel-titanium alloys to a self-expanding stentand other medical devices are disclosed in U.S. Pat. Nos. 4,665,906;5,067,957; 5,190,546; and 5,597,378 to Jervis and U.S. Pat. No.4,503,569 to Dotter. Another example is disclosed in European PatentApplication Publication No. EP0873734A2, entitled “Shape Memory AlloyStent.” This publication suggests a stent for use in a lumen in a humanor animal body having a generally tubular body formed from a shapememory alloy which has been treated so that it exhibits enhanced elasticproperties. The publication further suggests use of specified ternaryelements in a nickel-titanium alloy to obtain desired engineeringcharacteristics.

Use of a ternary element in a superelastic stent is also shown in, forexample, U.S. Pat. No. 5,907,893 to Zadno-Azizi et al. As a generalproposition, there have been attempts at adding a ternary element tonickel-titanium alloys as disclosed in, for instance, U.S. Pat. No.5,885,381 to Mitose et al.

Clearly, self-expanding, nickel-titanium stents are useful and valuableto the medical field. But a distinct disadvantage with self-expandingnickel-titanium stents is the fact that they are not sufficientlyradiopaque as compared to a comparable structure made from gold ortantalum. For example, radiopacity permits the cardiologist or physicianto visualize the procedure involving the stent through use offluoroscopes or similar radiological equipment. Good radiopacity istherefore a useful feature for self-expanding nickel-titanium stents tohave.

Radiopacity can be improved by increasing the strut thickness of thenickel-titanium stent. But increasing strut thickness detrimentallyaffects the flexibility of the stent, which is a quality necessary forease of delivery. Another complication is that radiopacity and radialforce co-vary with strut thickness. Also, nickel-titanium is difficultto machine and thick struts exacerbate the problem.

Radiopacity can be improved through coating processes such assputtering, plating, or co-drawing gold or similar heavy metals onto thestent. These processes, however, create complications such as materialcompatibility, galvanic corrosion, high manufacturing cost, coatingadhesion or delamination, biocompatibility, loss of coating integrityfollowing collapse and deployment of the stent, etc.

Radiopacity can also be improved by alloy addition. One specificapproach is to alloy the nickel-titanium with tungsten yet not lose theengineering benefits of the superelastic material. What has been neededand heretofore unavailable in the prior art is a superelasticnickel-titanium stent that applies tungsten to increase radiopacity yetpreserves the superelastic qualities of the nitinol.

SUMMARY OF THE INVENTION

The present invention is directed to a radiopaque stent for implantationin a body lumen and comprises a tubular-shaped body having a thin walldefining a strut pattern; wherein the body includes a nickel-titaniumsuperelastic alloy, and the alloy further includes a ternary elementincluding tungsten. As a result, the stent is highly radiopaque ascompared to a similar structure made of medical grade stainless steelthat is coated with a thin layer of gold.

Self-expanding nitinol stents are collapsed (that is, loaded) and thenconstrained within a delivery system. At the point of delivery, thestent is released (that is, unloaded) and allowed to return to itsoriginal diameter. The stent is designed to perform various mechanicalfunctions within the lumen, all of which are based upon the lowerunloading plateau stress. Therefore, it is crucial that the ternaryelement alloyed with the binary nickel-titanium does not diminish thesuperelastic characteristics of the nickel-titanium.

To achieve the sufficient degree of radiopacity yet maintaining thesuperelastic engineering properties of a binary nickel-titanium,preferably, the radiopaque stent of the present invention includestungsten whose atomic percent is greater than or equal to 5 and lessthan or equal to 12. In various alternative embodiments, the atomicpercent of the nickel is approximately 50.8, the atomic percent of thetitanium is a maximum of approximately 40, and the atomic percent of thetungsten is approximately 10.

With such compositions, the stress-strain hysteresis curve of thepresent invention radiopaque nitinol alloy closely approximates theidealized stress-strain hysteresis curve of binary nickel-titanium. Inother words, the present invention generally preserves the engineeringqualities of the nitinol alloy yet improves upon its radiopacity.

The present invention further contemplates a method for providing aradiopaque nitinol stent. In a preferred embodiment, the method entailsproviding a tubular-shaped body having a thin wall, wherein the bodyincludes a superelastic nickel-titanium alloy and the alloy furtherincludes tungsten; and forming a strut pattern; wherein the stent ishighly radiopaque. The step of providing a tubular-shaped body includesmelting nickel with titanium and tungsten and cooling to form an ingot,hot rolling the alloy, cold forming the alloy into a cylinder, drillingthe cylinder to form tubing, cold drawing the tubing, and annealing thetubing.

In a preferred embodiment, an austenite finish temperature (A_(f)) ofthe superelastic alloy in the stent is greater than or equal to zero andless than or equal to 30 degrees C. Also in the preferred embodiment,the Ni—Ti—W ingot prior to melting includes an austenite finishtemperature (A_(f)) of greater than or equal to 0 degrees C. and lessthan or equal to 20 degrees C. The Ni—Ti—W tubing includes an austenitefinish temperature (A_(f)) of greater than or equal to −15 degrees C.and less than or equal to 0 degrees C.

Other features and advantages of the present invention will become moreapparent from the following detailed description of the invention whentaken in conjunction with the accompanying exemplary drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, partially in section, depicting astent mounted on a delivery catheter and expanded within a damagedvessel, pressing a damaged vessel lining against the vessel wall.

FIG. 2 is a side elevational view, partially in section, depicting anexpanded stent within the vessel after withdrawal of the deliverycatheter.

FIG. 3 is an idealized stress-strain hysteresis curve for a superelasticmaterial.

FIG. 4 is a plan view of the flattened strut pattern of an exemplaryembodiment superelastic stent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to stents made of radiopaque nitinol. Thestents of the present invention can have virtually any configurationthat is compatible with the body lumen in which they are implanted. Thestent should preferably be configured so that there is a substantialamount of open area and preferably the open area to metal ratio is atleast 80 percent. The stent should also be configured so thatdissections or flaps in the body lumen wall are covered and tacked up bythe stent.

Referring to FIGS. 1, 2, and 4, in a preferred embodiment, a stent 10 ofthe present invention is formed partially or completely of alloys suchas nitinol (NiTi) which have superelastic (SE) characteristics. Stent 10is somewhat similar to the stent disclosed in U.S. Pat. No. 5,569,295,“Expandable Stents and Method for Making Same,” issued to Lam on Oct.29, 1996, which patent is incorporated herein by reference. Somedifferences of the present invention stent from that disclosed in the'295 patent is that the present invention stent is preferablyconstructed of a superelastic material with the addition of a ternaryelement, and the strut pattern has changed. Of course, the configurationof the stent 10 is just one example of many stent configurations thatare contemplated by the present invention.

Turning to FIG. 4, stent 10 preferably includes a plurality of radiallyexpandable cylindrical elements 24 disposed generally coaxially andinterconnected by members 26 disposed between adjacent cylindricalelements 24. The shapes of the struts 12 forming the pattern aredesigned so they can preferably be nested. This strut pattern is bestseen from the flattened plan view of FIG. 4. The serpentine shapedstruts 12 are nested such that the extended portions of the struts ofone cylindrical element 24 intrude into a complementary space within thecircumference of an adjacent cylindrical element. In this manner, theplurality of cylindrical elements 24 can be more tightly packedlengthwise.

As introduced above, an exemplary stent of the present inventionincludes a superelastic material. In a general sense, superelasticityimplies that the material can undergo a large degree of reversiblestrain as compared to common steel. In a technical sense, the term“superelasticity” and sometimes “pseudoelasticity” refer to anisothermal transformation in nitinol. More specifically, it refers tostress inducing a martensitic phase from an austenitic phase. Alloyshaving superelastic properties generally have at least two phases: amartensitic phase, which has a relatively low tensile strength and whichis stable at relatively low temperatures, and an austenitic phase, whichhas a relatively high tensile strength and which is stable attemperatures higher than the martensitic phase. Superelasticcharacteristics generally allow the metal stent to be deformed bycollapsing and deforming the stent and creating stress which causes theNiTi to reversibly change to the martensitic phase. The stent isrestrained in the deformed condition inside a delivery sheath typicallyto facilitate the insertion into a patient's body, with such deformationcausing the isothermal phase transformation. Once within the body lumen,the restraint on the stent is removed, thereby reducing the stressthereon so that the superelastic stent returns to its originalundeformed shape through isothermal transformation back to theaustenitic phase. Under these conditions, the stent can be described asself-expanding.

Returning to FIG. 1, the graphic illustrates, in a partialcross-sectional view, the distal end of a rapid exchange stent deliverysystem that includes a guide wire 14, a delivery sheath 16, and anintravascular catheter 18. For the sake of clarity, the illustration ofthe delivery system in FIG. 1 has been simplified. It is just oneexample of a delivery system that may be used with the presentinvention. More details of a delivery system specifically for use with aself-expanding stent may be found in, for example, U.S. Pat. No.6,077,295 to Limon et al., entitled “Self-Expanding Stent DeliverySystem,” which is incorporated herein by reference. Other deliverysystems such as over-the-wire may be used without departing from thescope of the instant invention.

FIG. 1 further shows an optional expandable balloon 20 inflated throughan inflation lumen (not shown). The stent 10 is first crimped on to thedeflated balloon 20, and the entire assembly is kept underneath thedelivery sheath 16 until the moment the stent 10 is deployed. The stent10 is self-expanding so that when the sheath 16 is withdrawn, the stent10 expands to its larger, deployment diameter without assistance fromthe balloon 20. Nevertheless, some procedures specifically use theballoon 20 to further expand the stent 10 for improved seating in theartery wall 29.

FIG. 2 illustrates the self-expanding stent 10 in the expanded conditionafter the delivery system has been removed. If an external force isapplied to the artery 28, the expanded stent 10 temporarily and at leastpartially collapses or deforms. As the stent 10 deforms, stress in thenickel-titanium alloy causes an isothermal phase transformation from theaustenitic phase to the martensitic phase. When the external force isremoved, the stress in stent 10 is likewise diminished so that the stentquickly transforms back from the martensitic phase to the austeniticphase. As this almost instantaneous, isothermal transformation occurs,the stent 10 returns to its fully expanded state and the artery remainsopen. When the superelastic stent 10 is implanted in an artery 28, itshigh resilience effectively maintains the patency of the artery whileminimizing the risk of permanent arterial collapse at the implant siteif the stent is temporarily deformed due to external forces.Furthermore, the resilience of the stent 10 supports the flap 30 tomaintain patency of the artery.

Stent 10 is preferably formed from a superelastic material such asnickel-titanium and undergoes an isothermal transformation when stressedif in the austenitic phase. For most purposes, the transformationtemperature for the stent 10 is preferably set low enough such that thenickel-titanium alloy is in the austenitic phase while at bodytemperature.

According to theory and practice, when stress is applied to a specimenof a metal such as nitinol exhibiting superelastic characteristics at atemperature at or above that which the transformation of the martensiticphase to the austenitic phase is complete, the specimen deformselastically until it reaches a particular stress level where the alloythen undergoes a stress-induced phase transformation from the austeniticphase to the martensitic phase. As the phase transformation progresses,the alloy undergoes significant increases in strain with little or nocorresponding increases in stress. The strain increases while the stressremains essentially constant until the transformation of the austeniticphase to the martensitic phase is complete (known as the “upperplateau”). Thereafter, further increase in stress is necessary to causefurther deformation. The martensitic metal first yields elastically uponthe application of additional stress and then plastically with permanentresidual deformation.

If the load on the specimen is removed before any permanent deformationhas occurred, the stress-induced martensite elastically recovers andtransforms back to the austenitic phase. The reduction in stress firstcauses a decrease in strain. As stress reduction reaches the level atwhich the martensitic phase begins to transforms back into theaustenitic phase, the stress level in the specimen remains essentiallyconstant (but less than the constant stress level at which theaustenitic crystalline structure transforms to the martensiticcrystalline structure until the transformation back to the austeniticphase is complete); i.e., there is significant recovery in strain withonly negligible corresponding stress reduction (known as the “lowerplateau”). After the transformation back to austenite is complete,further strain reduction results in stress reduction. This ability toincur significant strain at relatively constant stress upon theapplication of a load and to recover from the deformation upon theremoval of the load is commonly referred to as superelasticity andsometimes pseudoelasticity.

FIG. 3 illustrates an idealized stress-strain hysteresis curve for asuperelastic, binary nickel-titanium alloy. The relationship is plottedon x-y axes, with the x axis representing strain and the y axisrepresenting stress. For ease of illustration, the x-y axes are labeledon a scale typical for superelastic nitinol, with stress from 0 to 60ksi and strain from 0 to 9 percent, respectively.

Looking at the plot in FIG. 3, the line from point A to point Brepresents the elastic deformation of the nickel-titanium alloy. Afterpoint B the strain or deformation is no longer proportional to theapplied stress and it is in the region between point B and point C thatthe stress-induced transformation of the austenitic phase to themartensitic phase begins to occur. There also can be an intermediatephase, called the rhombohedral phase (or more commonly, the “R-Phase”),depending upon the composition and the thermomechanical history of thealloy.

At point C moving toward point D, the material enters a region ofrelatively constant stress with significant deformation or strain. Thisconstant stress region is known as the loading plateau, since itrepresents the behavior of the material as it encounters continuallyincreasing strain. It is in this plateau region C-D that thetransformation from austenite to martensite occurs.

At point D the transformation to the martensitic phase due to theapplication of stress to the specimen is substantially complete. Beyondpoint D the martensitic phase begins to deform, elastically at first,but, beyond point E, the deformation is plastic or permanent.

When the stress applied to the superelastic metal is removed, thematerial behavior follows the curve from point E to point F. Within theE to F region, the martensite recovers its original shape, provided thatthere was no permanent deformation to the martensitic structure. Atpoint F in the recovery process, the metal begins to transform from thestress-induced, metastable, martensitic phase back to the more stableaustenitic phase.

In the region from point G to point H, which is also an essentiallyconstant or plateau stress region, the phase transformation frommartensite back to austenite takes place. This constant stress regionG-H is known as the unloading plateau. The line from point Ito thestarting point A represents the elastic recovery of the metal to itsoriginal shape.

Binary nickel-titanium alloys that exhibit superelasticity have anunusual stress-strain relationship as just described and as plotted inthe curve of FIG. 3. As emphasized above, the superelastic curve ischaracterized by regions of nearly constant stress upon loading,identified above as loading plateau stress C-D and unloading plateaustress G-H. Naturally, the loading plateau stress C-D always has agreater magnitude than the unloading plateau stress G-H. The loadingplateau stress represents the period during which martensite is beingstress-induced in favor of the original austenitic crystallinestructure. As the load is removed, the stress-induced martensitetransforms back into austenite along the unloading plateau stress partof the curve. The difference in stress between the stress at loadingplateau C-D and unloading plateau G-H is the isothermal definition ofthe hysteresis in the system.

The present invention seeks to preserve the superelastic qualities ofnickel-titanium alloys just described yet improve upon the material'sradiopacity by addition of a ternary element. This is accomplished inone embodiment by forming a composition consisting essentially of about30 to about 52 atomic percent titanium and the balance nickel and up to10 atomic percent of one or more additional ternary alloying elements.The ternary alloy in the preferred embodiment is tungsten.

To achieve the sufficient degree of radiopacity yet maintaining thesuperelastic engineering qualities of a binary nickel-titanium,preferably, the radiopaque stent of the present invention includestungsten whose atomic percent is greater than or equal to 5 and lessthan or equal to 12. In various alternative embodiments, the atomicpercent of the nickel is approximately 50.8, the atomic percent of thetitanium is a maximum of approximately 40, and the atomic percent of thetungsten is approximately 10.

A preferred method of fabricating the present invention superelastic,radiopaque metallic stent entails first fashioning nickel-titaniumtubing. The tubing is made from vacuum induction melting nickel andtitanium with tungsten according to the composition suggested above. Theingot is then remelted for consistency. The ingot is next hot rolledinto bar stock, then straightened and sized, and cold formed into acylinder. The cylinder is gun drilled to form the tubing. Instead of gundrilling, other methods of material removal known in the art may beused, including electric discharge machining (EDM), laser beammachining, and the like. Next, the tubing is cold or hot drawn andannealed repeatedly to achieve the finished dimensions.

Any of the foregoing preferred embodiment steps may be repeated, takenout of sequence, or omitted as necessary depending on desired results.From here on, the tubing follows conventional stent fabricationtechniques such as laser cutting the strut pattern, heat setting, etc.

The following are additional guide posts for the nitinol processing toachieve a sufficiently radiopaque stent yet maintaining the superelasticstress-strain behavior of the alloy. In particular, empirical evidencesuggests that the Ni—Ti—W ingot should preferably have the followingaustenite finish temperature: 0 degrees C.≧A_(f)≦20 degrees C. TheNi—Ti—W tubing should exhibit an austenite finish of: −15 degreesC.≧A_(f)≦0 degrees C. In an exemplary embodiment, the final laser cutNi—Ti—W stent should exhibit an austenite finish temperature of: 0degrees C.≧A_(f)≦30 degrees C. Of course, the A_(f) of the finishedlaser cut stent can be set as needed by various heat treating processesknown in the art.

It is understood that the austenite finish temperature (A_(f)) isdefined to mean the temperature at which the material completely revertsto austenite. In technical terms, the A_(f) (and other transformationtemperatures A_(s), M_(s), M_(f)) as it applies to an ingot made ofNi—Ti—W, for example, is determined by a Differential ScanningCalorimeter (DSC) test, known in the art. The DSC test method todetermine transformation temperatures for the ingot is guided by ASTMstandard no. F 2004-00, entitled “Standard Test Method ForTransformation Temperature Of Nickel-Titanium Alloys By ThermalAnalysis.”

The “active A_(f)” for the tubing and the finished stent is determinedby a bend and free recovery test, also known in the art. In such a test,the tubing is cooled to under the M_(f) temperature, deformed, andwarmed up. While monitoring the increasing temperature, the point offinal recovery of the deformation in the tubing approximates the A_(f)of the material. The active A_(f) testing technique is guided by asecond ASTM standard entitled “Standard Test Method For Determination OfTransformation Temperature Of Nickel-Titanium Shape Memory Alloys ByBend And Free Recovery,” or by equivalent test methods known in the art.

The present invention further provides a nitinol stent having improvedradiopacity without resorting to increasing the wall thickness or strutthickness. Increasing wall or strut thicknesses detracts from theflexibility of the stent, which is detrimental to deliverability.Rather, the present invention superelastic nitinol stent has a thinwall/strut thickness and/or strut cross-sectional area akin to aconventional stainless steel stent, and has comparable radiopacity to astainless steel stent with a thin coating of gold. The wall/strutthickness is defined by the difference between the inside diameter andthe outside diameter of the tube. In an exemplary embodiment of thepresent invention, for an approximately 21 mm long stent with anexpanded diameter of about 8 mm, the wall thickness is approximately0.0045 inch. If the exemplary embodiment stent strut has a square shapecross-sectional area, its dimensions would be 0.0045 inch by 0.0045 inchwith a cross-sectional area of approximately 0.000020 in².

Another aspect of nitinol aside from its superelasticity is shapememory. Sometimes, these two features of nitinol are loosely describedas two sides of the same coin. The present invention can also beemployed with respect to this “thermoelastic” physical attribute asdescribed below.

The shape memory effect allows a nitinol structure to be deformed tofacilitate its insertion into a body lumen or cavity, and then heatedwithin the body so that the structure returns to its original, setshape. Nitinol alloys having shape memory effect generally have at leasttwo phases: a martensitic phase, which has a relatively low tensilestrength and which is stable at relatively low temperatures, and anaustenitic phase, which has a relatively high tensile strength and whichis stable at temperatures higher than the martensitic phase.

Shape memory effect is imparted to the alloy by heating thenickel-titanium metal to a temperature above which the transformationfrom the martensitic phase to the austenitic phase is complete; i.e., atemperature above which the austenitic phase is stable. The shape of themetal during this heat treatment is the shape “remembered.” Theheat-treated metal is cooled to a temperature at which the martensiticphase is stable, causing the austenitic phase to transform to themartensitic phase. The metal in the martensitic phase is thenplastically deformed, e.g., to facilitate the entry thereof into apatient's body. Subsequent heating of the deformed martensitic phase toa temperature above the martensite to austenite transformationtemperature causes the deformed martensitic phase to transform to theaustenitic phase. During this phase transformation the metal revertsback to its original shape.

The recovery or transition temperature may be altered by making minorvariations in the composition of the metal and in thermomechanicalprocessing the material. In developing the correct composition,biological temperature compatibility must be determined in order toselect the correct transition temperature. In other words, when thestent is heated, it must not be so hot that it is incompatible with thesurrounding body tissue. Other shape memory materials may also beutilized, such as, but not limited to, irradiated memory polymers suchas autocrosslinkable high density polyethylene (HDPEX). Shape memoryalloys are known in the art and are discussed in, for example, “ShapeMemory Alloys,” Scientific American, Vol. 281, pp. 74-82 (November1979), incorporated herein by reference.

Shape memory alloys undergo a transition between an austenitic phase anda martensitic phase at certain temperatures. When they are deformedwhile in the martensitic phase, they retain this deformation as long asthey remain in the same phase, but revert to their originalconfiguration when they are heated to a transition temperature, at whichtime they transform to their austenitic phase. The temperatures at whichthese transitions occur are affected by the nature of the alloy and thethermomechanical history of the material. Nickel-titanium-based alloys(NiTi), wherein the transition temperature is slightly lower than bodytemperature, are preferred for the present invention. It is desirable tohave the transition temperature set at just below body temperature toinsure a rapid transition from the martinsitic state to the austeniticstate when the stent is implanted in a body lumen.

Turning again to FIGS. 1, 2, and 4, the present invention stent 10 isformed from a shape memory alloy, such as NiTi discussed above. Afterthe stent 10 is inserted into an artery 28 or other vessel, the deliverysheath 16 is withdrawn exposing the stent 10 to the ambient environment.The stent 10 then immediately expands due to contact with the highertemperature within artery 28 as described for devices made from shapememory alloys. An optional expandable balloon 20 may be inflated byconventional means to further expand the stent 10 radially outward.

Again, if an external force is exerted on the artery, the stent 10temporarily at least partially collapses. But the stent 10 then quicklyregains its former expanded shape due to its shape memory qualities.Thus, a crush-resistant stent, having shape memory characteristics, isimplanted in a vessel. It maintains the patency of a vessel whileminimizing both the risk of permanent vessel collapse and the risk ofdislodgment of the stent from the implant site if the stent istemporarily deformed due to external forces.

When the stent 10 is made in accordance with the present invention, itis also highly radiopaque. The same alloying processes described earlierare used here to add the ternary element to increase the radiopacity ofthe stent. Insofar as the martensitic to austenitic phase transformationis thermally driven, the deployment of the present invention stent canbe explained in terms of the shape memory effect.

While the present invention has been illustrated and described herein interms of a radiopaque nitinol stent, it is apparent to those skilled inthe art that the present invention can be used in other instances. Othermodifications and improvements may be made without departing from thescope of the present invention.

What is claimed is:
 1. An endoprosthesis delivery system comprising: adelivery sheath; a balloon catheter configured to be disposed within thedelivery sheath, the balloon catheter including an inflatable balloon;and a endoprosthesis disposed on the inflatable balloon, theendoprosthesis comprising: a tubular-shaped body, the body including anickel-titanium superelastic alloy, and the alloy further includes aternary element including tungsten, wherein the tubular-shaped bodyincludes a nickel-titanium-tungsten tubing having an austenite finishtemperature of −15 degrees C. to 0 degrees C.
 2. The endoprosthesisdelivery system of claim 1, wherein the atomic percent of the tungstengreater than or equal to 5 and less than or equal to
 12. 3. Theendoprosthesis delivery system of claim 1, wherein the atomic percent ofthe nickel is approximately 50.8, the atomic percent of the titanium isapproximately 40, and the atomic percent of the tungsten isapproximately
 10. 4. The endoprosthesis delivery system of claim 1,wherein the superelastic alloy includes a stress-induced martensitephase.
 5. The endoprosthesis delivery system of claim 1, wherein theatomic percent of the nickel is approximately 50.8, the atomic percentof the titanium is a maximum of approximately 40, and the atomic percentof the tungsten is approximately
 10. 6. The endoprosthesis deliverysystem of claim 1, wherein an austenite finish temperature (A_(f)) ofthe superelastic alloy in the endoprosthesis is greater than or equal tozero and less than or equal to 30 degrees C.
 7. The endoprosthesisdelivery system of claim 1, wherein the tubular-shaped body includes rawtubing having an austenite finish temperature (A_(f)) of greater than orequal to −15 degrees C. and less than or equal to 0 degrees C.
 8. Theendoprosthesis delivery system of claim 1, wherein the endoprosthesishas a greater radiopacity than a 316L stainless steel endoprosthesiscoated with 4.7 μm of gold.
 9. A method of implanting an endoprosthesis,the method comprising: positioning an endoprosthesis delivery systemwithin a body lumen, the endoprosthesis delivery system comprising adelivery sheath and an endoprosthesis disposed within the deliverysheath, the endoprosthesis comprising: a tubular-shaped body including anickel-titanium superelastic alloy, and the alloy further includes aternary element including tungsten, wherein the tubular-shaped bodyincludes a nickel-titanium-tungsten tubing having an austenite finishtemperature of −15 degrees C. to 0 degrees C.; and withdrawing thedelivery sheath of the endoprosthesis delivery system to expose anendoprosthesis and deploy the endoprosthesis into the body lumen; andwithdrawing the delivery sheath from the body lumen.
 10. The method ofclaim 9, further comprising positioning a guidewire within the bodylumen and advancing the endoprosthesis delivery system along theguidewire to the body lumen.
 11. The method of claim 9, furthercomprising positioning a balloon adjacent the endoprosthesis.
 12. Themethod of claim 11, further comprising expanding the balloon to expandthe endoprosthesis within the body lumen.
 13. The method of claim 11,wherein the endoprosthesis has a greater radiopacity than a 316Lstainless steel endoprosthesis coated with 4.7 μm of gold.
 14. Anendoprosthesis delivery system comprising: a delivery sheath; a ballooncatheter configured to be disposed within the delivery sheath, theballoon catheter including an inflatable balloon; and an endoprosthesisdisposed on the inflatable balloon, the endoprosthesis comprising: atubular-shaped body wherein the body includes a nickel-titaniumsuperelastic alloy with a ternary element including tungsten, said alloyexhibiting a stress-strain curve that approximates the idealizedstress-strain curve of binary nickel-titanium, wherein the body isradiopaque and the tubular-shaped body includes a pattern with struts nogreater than 0.0046 inch in width.
 15. The endoprosthesis deliverysystem of claim 14, wherein the atomic percent of the tungsten greaterthan or equal to 5 and less than or equal to
 12. 16. The endoprosthesisdelivery system of claim 14, wherein the atomic percent of the nickel isapproximately 50.8, the atomic percent of the titanium is approximately40, and the atomic percent of the tungsten is approximately
 10. 17. Theendoprosthesis delivery system of claim 14, wherein the superelasticalloy includes a stress-induced martensite phase.
 18. The endoprosthesisdelivery system of claim 14, wherein the atomic percent of the nickel isapproximately 50.8, the atomic percent of the titanium is a maximum ofapproximately 40, and the atomic percent of the tungsten isapproximately
 10. 19. The endoprosthesis delivery system of claim 14,wherein the superelastic alloy in the tubing with said strut pattern hasan austenite finish temperature (A_(f)) of greater than or equal to zeroand less than or equal to 30 degrees C.
 20. The endoprosthesis deliverysystem of claim 14, wherein the tubular-shaped body includes raw tubinghaving an austenite finish temperature (A_(f)) of greater than or equalto −15 degrees C. and less than or equal to 0 degrees C.
 21. A method ofimplanting an endoprosthesis, the method comprising: positioning anendoprosthesis delivery system within a body lumen, the endoprosthesisdelivery system comprising a delivery sheath and an endoprosthesisdisposed within the delivery sheath, the endoprosthesis comprising: atubular-shaped body wherein the body includes a nickel-titaniumsuperelastic alloy with a ternary element including tungsten, said alloyexhibiting a stress-strain curve that approximates the idealizedstress-strain curve of binary nickel-titanium, wherein the body isradiopaque and the tubular-shaped body includes a strut pattern withstruts no greater than 0.0046 inch in width; and withdrawing thedelivery sheath of the endoprosthesis delivery system to expose anendoprosthesis and deploy the endoprosthesis into the body lumen; andwithdrawing the delivery sheath from the body lumen.
 22. The method ofclaim 21, further comprising positioning a guidewire within the bodylumen and advancing the endoprosthesis delivery system along theguidewire to the body lumen.
 23. The method of claim 21, furthercomprising positioning a balloon adjacent the endoprosthesis.
 24. Themethod of claim 21, further comprising expanding the balloon to expandthe endoprosthesis within the body lumen.
 25. An endoprosthesis deliverysystem comprising: a delivery sheath; a balloon catheter configured tobe disposed within the delivery sheath, the balloon catheter includingan inflatable balloon; and an endoprosthesis disposed on the inflatableballoon, the endoprosthesis comprising: a tubular-shaped body whereinthe body includes a nickel-titanium superelastic alloy with a ternaryelement including tungsten, wherein the nickel-titanium superelasticalloy includes: 30 to 52 atomic percent titanium, 5 to 12 atomic percenttungsten, and a balance of nickel; wherein the tubular-shaped body isformed from a nickel-titanium-tungsten tubing having an austenite finishtemperature (A_(f)) of −15 degree C. to 0 degree C., wherein the body isradiopaque.
 26. The endoprosthesis delivery system of claim 25, whereinthe nickel-titanium superelastic alloy includes 50.8 atomic percentnickel, 10 atomic percent tungsten, and a balance of titanium.
 27. Theendoprosthesis delivery system of claim 25, wherein the tubing includesa strut pattern with struts no greater than 0.0046 inch in width. 28.The endoprosthesis delivery system of claim 25, wherein the superelasticalloy includes a stress-induced martensite phase.
 29. The endoprosthesisdelivery system of claim 25, wherein an austenite finish temperature(A_(f)) of the superelastic alloy in the tubing endoprosthesis isgreater than or equal to zero and less than or equal to 30 degrees C.30. A method of implanting an endoprosthesis, the method comprising:positioning an endoprosthesis delivery system within a body lumen, theendoprosthesis delivery system comprising a delivery sheath and anendoprosthesis disposed within the delivery sheath, the endoprosthesiscomprising: a tubular-shaped body wherein the body includes anickel-titanium superelastic alloy with a ternary element includingtungsten, wherein the nickel-titanium superelastic alloy includes: 30 to52 atomic percent titanium, 5 to 12 atomic percent tungsten, and abalance of nickel; wherein the tubular-shaped body is formed from anickel-titanium-tungsten tubing having an austenite finish temperature(A_(f)) of −15 degree C. to 0 degree C., wherein the body is radiopaque;and withdrawing the delivery sheath of the endoprosthesis deliverysystem to expose an endoprosthesis and deploy the endoprosthesis intothe body lumen; and withdrawing the delivery sheath from the body lumen.31. The method of claim 30, further comprising positioning a guidewirewithin the body lumen and advancing the endoprosthesis delivery systemalong the guidewire to the body lumen.
 32. The method of claim 30,further comprising positioning a balloon adjacent the endoprosthesis.33. The method of claim 32, further comprising expanding the balloon toexpand the endoprosthesis within the body lumen.
 34. An endoprosthesisdelivery system comprising: a delivery sheath; a balloon catheterconfigured to be disposed within the delivery sheath, the ballooncatheter including an inflatable balloon; and an endoprosthesis disposedon the inflatable balloon, the endoprosthesis comprising: atubular-shaped body including: an atomic percent of nickel of 50.8, anatomic percent of tungsten greater than or equal to 5 and less than orequal to 12, and a balance of titanium wherein the tubular-shaped bodyhas an active austenite finish temperature A_(f) defined by −15 degreeC.≦active A_(f)≦0 degree C.
 35. The endoprosthesis delivery system ofclaim 34, wherein the tubular-shaped body includes 50.8 atomic percentnickel, 10 atomic percent tungsten, and a balance of titanium.
 36. Theendoprosthesis delivery system of claim 34, wherein the tubing includesa strut pattern having struts and each strut has a cross-sectional areaof less than 0.000021 in².
 37. A method of implanting an endoprosthesis,the method comprising: positioning an endoprosthesis delivery systemwithin a body lumen, the endoprosthesis delivery system comprising adelivery sheath and an endoprosthesis disposed within the deliverysheath, the endoprosthesis comprising: a tubular-shaped body wherein thebody includes a nickel-titanium superelastic alloy with a ternaryelement including tungsten, wherein the nickel-titanium superelasticalloy includes: 30 to 52 atomic percent titanium, 5 to 12 atomic percenttungsten, and a balance of nickel; wherein the tubular-shaped body isformed from a nickel-titanium-tungsten tubing having an austenite finishtemperature (A_(f)) of −15 degree C. to 0 degree C., wherein the body isradiopaque; and withdrawing the delivery sheath of the endoprosthesisdelivery system to expose an endoprosthesis and deploy theendoprosthesis into the body lumen; and withdrawing the delivery sheathfrom the body lumen.
 38. The method of claim 37, further comprisingpositioning a guidewire within the body lumen and advancing theendoprosthesis delivery system along the guidewire to the body lumen.39. The method of claim 37, further comprising positioning a balloonadjacent the endoprosthesis.
 40. The method of claim 39, furthercomprising expanding the balloon to expand the endoprosthesis within thebody lumen.